1. Field of the Invention
The present invention relates to a water jet propulsion watercraft, and more specifically, to a water jet propulsion watercraft that monitors whether a catalytic converter is functioning effectively.
2. Description of the Related Art
Conventionally, a water jet propulsion watercraft is provided with an oxygen sensor for detecting an oxygen concentration of an exhaust gas. For example, a water jet propulsion watercraft disclosed in Laid-open Japanese Patent Application No. 2006-64425 controls an air-fuel ratio of an air-fuel mixture based on an oxygen concentration detected by an oxygen sensor. The water jet propulsion watercraft has an exhaust passage that guides exhaust gas from an engine to an exterior of a hull of the watercraft. A catalytic converter is arranged in the exhaust passage. The oxygen sensor is arranged in the exhaust passage at a position upstream of the catalytic converter.
Laid-open Japanese Patent Application No. H09-184462 discloses a water jet propulsion watercraft in which an oxygen sensor is arranged in a gas collection chamber. The gas collection chamber is connected to a cylinder of an engine. Laid-open Japanese Patent Application No. H09-310630 discloses a water jet propulsion watercraft in which an oxygen sensor is installed in an exhaust probe installation hole. The exhaust probe installation hole is arranged in the exhaust system at a position upstream of a water lock. The exhaust probe installation hole is arranged below and near a maintenance opening. Laid-open Japanese Patent Application No. H11-013569 discloses an oxygen sensor mounting structure for a small boat in which an oxygen sensor is arranged in a volume chamber. The volume chamber is connected to an exhaust passage or a cylinder hole of an engine. Laid-open Japanese Patent Application No. H11-079092 discloses an exhaust apparatus for a boat in which a plurality of exhaust pipes are connected to an engine. The downstream ends of the exhaust pipes are connected to a collector chamber. The oxygen sensor is arranged in the collector chamber. Laid-open Japanese Patent Application No. H11-245895 discloses an exhaust apparatus for a small planing boat in which an oxygen sensor is arranged upstream of a catalytic converter in an exhaust pipe. Additionally, an exhaust temperature sensor is arranged downstream of the catalytic converter in the exhaust pipe. Laid-open Japanese Patent Application No. 2001-200746 discloses an exhaust system for a small boat in which a mounting hole is formed in an exhaust pipe. When the boat is operated during an outgoing inspection, an A/F sensor is installed in the mounting hole. A feedback control is executed based on a detection result obtained with the A/F sensor, and a revision value of a fuel injection amount is determined with respect to a target air-fuel ratio and stored. After the determined revision value is stored, the A/F sensor is removed and the mounting hole is blocked with a bolt. Laid-open Japanese Patent Application No. 2003-205896 discloses an exhaust apparatus for a water jet propulsion watercraft in which an independent exhaust passage extends from each exhaust port of a cylinder block having a plurality of exhaust ports. An oxygen sensor is installed in an upper portion of at least one of the independent exhaust passages.
In recent years, there has been a demand to reduce the amount of harmful substances contained in exhaust gases discharged from engines in consideration of the impact such substances have on the environment. With the water jet propulsion watercraft of Laid-open Japanese Patent Application No. 2006-64425, the amount of harmful substances in the exhaust gas can be reduced by the catalytic converter arranged in the exhaust passage. However, there are situations in which the catalytic converter does not function effectively due to degradation or other causes. In such a case, exhaust gas will be discharged to the outside without having the harmful substances sufficiently reduced. Therefore, it is important to monitor if the catalytic converter is functioning effectively. In the water jet propulsion watercraft disclosed in Laid-open Japanese Patent Application No. 2006-64425, the oxygen sensor is positioned upstream of the catalytic converter. Consequently, it is not possible to monitor whether the catalytic converter is functioning effectively based on a detection result from the oxygen sensor. A similar problem exists in the other mentioned documents because either the oxygen sensor is positioned upstream of the catalytic converter or a catalytic converter is not even provided.